Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of undesired vapor emissions that could release fuel vapors to the atmosphere. Undesired vapor emissions may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system may increase if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. A pressure rise above a threshold may indicate the absence of undesired fuel system vapor emissions. Alternatively, in the absence of a pressure rise above a threshold, as a fuel system cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation may monitored and undesired fuel system vapor emissions identified based on expected vacuum development or expected rates of vacuum development.
Entry conditions and thresholds for an EONV test may be based on an inferred total amount of heat rejected into the fuel tank during the prior drive cycle. The inferred amount of heat may be based on engine run-time, integrated mass air flow, fuel level, ambient temperature, reid vapor pressure, etc. While these heat rejection inferences work well in most conditions, they may be prone to errors when noise factors are involved. For example, if a vehicle is driven downhill for an extended period, driven under rainy and/or windy conditions, or under conditions where a period of high-speed driving is followed by a period of idling, much of the heat rejection to the fuel tank may be negated. As a result, in an example where an EONV test is executed based on a heat rejection inference where the above-described noise factors are involved may result in a false failure. Furthermore, relying solely on heat rejection for conducting EONV diagnostics may be problematic for hybrid vehicles, where engine run-time may be limited, thus limiting an amount of heat rejected from the engine for particular drive cycles.
Still further, a typical EONV test may be enabled to run for a predetermined time duration (e.g. 45 minutes), where the time limit may be a function of battery power. Accordingly, if a vehicle initiates an EONV test at a vehicle-off event, but the vehicle does not remain in a vehicle-off state for more than the predetermined EONV time duration, then the EONV test may be aborted. EONV tests that are executed, but not completed, may affect completion rates, and may further result in increased loading of a fuel vapor canister, increased wear and tear on valves that are actuated open or closed to conduct the EONV test, and may further result in some examples in premature fuel pump shutoffs. Thus, a more intelligent means of determining when and how to execute diagnostic tests for undesired evaporative emissions, is desired.
The inventors have herein recognized these issues, and have developed systems and methods to at least partially address the above issues. In one example, a method is provided, comprising learning routes commonly traveled by an engine-driven vehicle, including altitude changes and stop durations. The method may further include storing fuel vapors from a fuel system supplying fuel to the engine in a vapor storage device positioned in an evaporative emissions system, and diagnosing the fuel system and evaporative emissions system for undesired evaporative emissions based on a learned altitude change in a first condition, and based on a learned stop duration in a second condition.
As one example, the learned altitude change in the first condition includes a change in altitude sufficient to result in a pressure change yield in the fuel system and evaporative emissions system greater than a predetermined pressure change yield threshold, under conditions where the fuel system and evaporative emissions system are sealed from atmosphere during the change in altitude. Responsive to the predetermined pressure change yield threshold being reached during the change in altitude, the method may include sealing or maintaining sealed the fuel system and evaporative emissions system, monitoring pressure in the sealed fuel system and evaporative emissions system, and indicating the fuel system and evaporative emissions system are free from undesired evaporative emissions responsive to pressure in the fuel system and evaporative emissions system not reaching a predetermined pressure threshold for a predetermined duration.
As one example, diagnosing the fuel system and evaporative emissions system based on a learned stop duration in the second condition further comprises responsive to a vehicle-off event corresponding to the learned stop duration comprising a duration less than a predetermined threshold duration, actively reducing pressure in the fuel system and evaporative emissions system to a predetermined target vacuum, and actively maintaining the predetermined target vacuum until a vehicle-off event is indicated. Responsive to the predetermined target vacuum being reached and the vehicle-off event being indicated, the method may include sealing or maintaining sealed the fuel system and evaporative emissions system from engine intake and from atmosphere. Subsequently, the method may include indicating an absence of undesired evaporative emissions in the fuel system and evaporative emissions system responsive to pressure bleed-up in the fuel system and evaporative emissions system not reaching a predetermined pressure bleed-up threshold, or responsive to a pressure bleed-up rate not reaching or exceeding a predetermined pressure bleed-up rate threshold.
In this way, a vehicle fuel system and evaporative emissions system may be diagnosed as to the presence or absence of undesired evaporative emissions, where the tests may be scheduled according to learned travel routines. By scheduling the tests based on learned travel routines, the tests may be initiated at times wherein the tests are likely to be completed without prematurely being aborted, which may increase completion rates and which may reduce undesired evaporative emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.